1. Field of the Invention
This invention generally relates to the area of electronic test equipment. More specifically, it pertains to devices that are capable of measuring xe2x80x9ctransfer functionsxe2x80x9d, i.e., functions that are an indication of a system""s response to electromagnetic radiation. These test devices are commonly known as Network Analyzers.
2. Description of the Prior Art
We live in a sea of electromagnetic radiation. If we could visualize the radiation swirling about us, we would see every electrical circuit and every electrical system putting out its own radiation. Unfortunately, some of this radiation affects equipment that we depend on, such as our office computers, computer-controlled machines, and radio and television equipment. It is important, therefore, to be able to explore the radiation that will possibly impact our equipment and predict whether the equipment will be adversely affected by the radiation or whether the steps we have taken to protect the equipment, such as using conducting seals and grounding devices, will provide the necessary protection.
As our society becomes more laden with electronic wizardry, this task becomes even more important. Will your computer shut down when you turn on your new cellular telephone? Will your pacemaker malfunction when you turn on the microwave oven? With respect to electronic-intenisive equipment, such as aircraft and water craft navigational instruments, would nearby lightning or operating a computer on board adversely affect the usefulness of the equipment?
In order for industry to warrant the operation of their products, such as computers, computer-controlled machines, navigation instruments, cellular telephones and the like, the effects of external radiation on the particular device must be determined, and procedures or measures taken to prevent this radiation from corrupting the operation of the device. In order to market many electronic devices in the United States, the Federal Communications Commission (FCC) rules require certification against the external electromagnetic radiation effects. European Union rules and regulations require more stringent certification requirements for Electromagnetic Interference and Compatibility (EMI/EMC) applicable to all consumer and industrial equipment.
The interference caused by electromagnetic radiation can be characterized by measuring a quantity called. xe2x80x9ctransfer functionxe2x80x9d also known as xe2x80x9ctransfer characteristicsxe2x80x9d or xe2x80x9cimpulse responsesxe2x80x9d. Transfer functions, in general, characterize a system by relating the system""s response (output) to a given excitation (input). This is depicted in FIG. 1. Transfer function determination requires one signal (for exciting the system under test) and two measurements (the excitation and response). In FIG. 1, the excitation source or signal 1 (such as electromagnetic or acoustic radiation) is inputted to the system 2 under test (such as an aircraft, a computer, or a building) to bring about a response 3 (such as current or voltage induced on a cable or acoustic intensity in a chamber). The transfer function is defined as the ratio of the output signal response 3 to the input signal 1, and must be determined for each excitation frequency. For example, a piano""s transfer function can be determined by striking the keys. To make a complete transfer function, the piano response must be determined for each and every key; each key representing a different frequency. Examples of practical applications for transfer function measurements are to:
a) Characterize electronic filters;
b) Characterize mechanical vibrations of structures (such as automobile suspensions or bridges);
c) Characterize concert hall echoes;
d) Determine acoustic shielding effectiveness (blockage of sound) of enclosures; and,
e) Determine electromagnetic shielding effectiveness of automobiles, aircraft, buildings, missiles etc . . . .
The prior art utilizes two primary techniques in conjunction with a centralized data to acquisition and processing apparatus for measuring these transfer functions:
High-Pitier/Ultra-Wideband Test Technique
The high-power/ultra-wideband technique excites the system with a short duration pulse (called an xe2x80x9cimpulsexe2x80x9d). Because any impulse has a frequency content that is extremely rich, i.e., contains many frequencies, one measurement contains transfer function information over a wide band of frequencies. The system transfer function is determined by computing the system""s response to the impulse using a data processing technique such as the Fourier transformation technique. This is a fast measurement technique because the transfer function can be determined over a large frequency range using only one pulse.
Impulse excitation does not use up-conversion techniques to modulate the wideband pulse onto a carrier frequency. The pulse is used as the direct excitation source and the frequency range of the measurement is inversely proportional to the duration of the impulse. Therefore, increasing the frequency range of the measurement requires decreasing the impulse width. This places a limit on the maximum frequency that this method can be used because of practical limitations in narrowing the pulse due to electronic component limitations.
The processing method used for this technique (Fourier transform) does not give any data quality indication. This requires the operator to set the excitation power to levels that are higher than necessary in order to guarantee that noise does not corrupt the data. High-power amplifiers are required to boost the signal power of the impulse, because the transmitted pulse has a very short duration.
There are several drawbacks to this high-power/ultra-wideband method:
a) High-power amplifiers are expensive;
b) Ultra-wideband electronic components are unavailable or expensive;
c) High-power pulses can be hazardous to humans;.
d) High-power pulses can cause interference with other electronic systems;
e) The maximum frequency range is inherently limited; and,
f) Noise, which is a practical consideration in all measurements, can corrupt the transfer function, sometimes without the operator""s knowledge.
Low Power/Narrow band or Continuous Wave (CW) Test Technique
The low-power/narrowband technique, commonly referred to as continuous wave (CW) testing, is the most popular technique used. This technique excites the system with a signal that is comprised of one dominant frequency (called a xe2x80x9ctonexe2x80x9d). This is similar to one piano key struck and your ear hearing only one dominant tone. The system transfer function is determined by measuring the system""s response to the tone. A complete transfer function requires a tone to be transmitted at each and every frequency of interest.
The processing method used for this technique does not give any data quality indication. This requires the operator to set the excitation power to levels that are higher than necessary in order to guarantee that noise does not corrupt the data.
There are several drawbacks to this low-power/narrowband method:
a) Testing over a large frequency span is very slow due to the excessive number of individual tones that are required;
b) Transmitting tones can cause interference with other electronic systems;
c) Vital transfer function information can be missed due to the frequency stepping nature of the testing sequence; and,
d) Noise, which is a practical consideration in all measurements, can corrupt the transfer function, sometimes without the operator""s knowledge.
Centralized Data Acquisition and Processing Apparatus
For large and complex systems under test, the prior art utilizes a centralized data acquisition and processing apparatus to implement the standard transfer function setup shown in FIG. 2.
A signal generator 4 generates a controlled excitation signal 1 and sends it via an antenna 5 to radiate the system 2 under test to produce the system response 3. An amplifier (not shown) may be used to increase the strength of the excitation signal. Sensors 6 and 7 (e.g., field probes, current and voltage sensors or acoustic transducers) detect the input signal and the output response signal respectively and send them via analog telemetry links 8 (e.g., wideband analog fiber optic cables) to receivers 9 (e.g., high speed digitizers, Network Analyzers, Spectrum Analyzers) to simultaneously monitor and record the excitation as well as the system response signals. Signal generator 4 and receivers 9 are usually located in a centralized location away from the system 2 under test in a protected enclosure 10 shielded from the collateral effects of the RF radiation as radiated by the antenna 5 as well as other ambient radiation sources. While the currents or voltages are sensed at the desired locations in the system under test, they are actually recorded at the centrally located receivers.
When the high-power/ultra-wideband technique is used, the transmitter or signal generator 4, consisting of a wideband high amplitude pulser; the receivers 9, which typically consist of high speed digitizers; and the data acquisition computer 14 are located together in the centralized shielded enclosure. The wideband transmitting antenna 5 is located outside the shielded enclosure 10. The sensed excitation and system responses are relayed in analog form to the centrally located receivers 9 via links 8 which have utilized analog microwave or fiber optic telemetry technologies. While the currents or voltages are sensed at the desired location in the system under test, they are actually recorded at the centrally located receivers 9 (high speed digitizers).
When the low-power/narrowband or CW technique is utilized, the computer 14, signal generator 4 and two receivers 9 are built into a single commercially available instrument such as a Network Analyzer. This instrument typically contains one source signal output port, and two input ports for the receivers. Usually a third receiver port is included to enable simultaneous measurement of a reference signal, if desired. The generated source signal is relayed to a radiation device 5 located outside the shielded enclosure 10, and an amplifier is generally used to increase the signal level as necessary to produce measurable signal levels in the system 2 under test. Again, while the currents or voltages are sensed at the desired location in the system under test, they are actually recorded at the centrally located receivers (Network Analyzer).
There are several drawbacks to this centralized data acquisition and processing:
a) Sensor data can only be relayed from the test point location to the receiver using analog telemetry because the signal is not recorded until it arrives at the receiver input port;
b) Analog telemetry links have a relatively small dynamic range, typically 40 dB, which reduces the fidelity of the desired sensed signals;
c) Attenuators or preamplifiers must be used to adjust the sensed signal until its peak amplitude is within the linear range of the analog telemetry link;
d) It takes a long time to measure a transfer function over the full frequency range of interest because the Network Analyzer must dwell longer at each frequency to make up for the poor dynamic range of the analog telemetry links;
e) Analog fiber optic telemetry links with operational bandwidth of hundreds of MHz are very expensive due, in part, to the high cost of wideband fiber optic cable. The analog fiber optic telemetry links with bandwidths exceeding 1 GHz are extremely expensive, highly unreliable and not widely available; and,
f) The number of receivers that simultaneously can reliably measure frequency and phase-locked responses at multiple test points (locations) within the system under test is limited to three received channels in a common Network Analyzer instrument.
U.S. Pat. No. 5,086,616 xe2x80x9cMonitoring Process and Dice of a Quasi-Closed Electromagnet Shieldxe2x80x9d discloses a measurement system, however, the source of the radiation is retained inside the enclosure, the frequency range is limited to 1 to 100 KHz, and the source is low power.
This invention is a low-power/wideband (LPWB) transfer function measurement method and a distributed data acquisition and processing apparatus with which to perform the aforesaid method. The method uses a completely new approach in the artxe2x80x94Stochastic Process Test Technique(SPTT)xe2x80x94for deriving transfer functions and a distributed, rather than centrally located, instrumentation apparatus to manage the test process.
In the invention, an advanced stochastic processing test technique (SPTT) enables determination of transfer functions using LPWB-schemes. The system is excited by being radiated with a low power pseudo-random signal that has a concentrated frequency content over a wide, continuous band (wideband). The bandwidth of this signal can be Lip to several thousand times larger than a CW tone. Therefore, one LPWB pulse can excite the system with the equivalent of thousands of CW tones. This has the advantage of greatly decreasing the time required to make transfer function measurements. Interference with other electronic devices is considerably reduced, because the low power excitation signal has a structure that is similar to noise. Since the excitation is over a continuous band, as opposed to CW which changes frequency in discrete steps, detailed transfer function information is not missed.
LPWB uses a wideband excitation baseband signal and an up-conversion process to operate at frequencies that are greater than the highest modulation frequency of the baseband. The up-conversion process shifts the baseband excitation signal upward to the various frequency bands of interest. By using this method, transfer function determination can be made at frequencies above the baseband modulation frequency, thus removing the limitation associated with impulse testing, while making measurements across a wide frequency range, thus improving on the speed limitation of CW testing. The receivers use a corresponding down-conversion process to demodulate the received signal to the original baseband modulation bandwidth before recording. This down-conversion technique allows low cost, low frequency electronic components to be used for recording and telemetry needs.
Low-power excitation measurements are possible because of the excitation signal duration and the SPTT transfer function data processing method used. A high-power short duration pulse has the equivalent average power of a low-power, long duration pulse. Because the LPWB excitation signal has a duration. which is much longer than an impulse""s duration, measurements can be made at low-power when compared to the high-power/ultra-wideband test technique. Additionally, although the LPWB excitation signal has a significantly wider bandwidth than the CW excitation signal, such that the LPWB radiated power is distributed over. the entire frequency bandwidth resulting in a much lower power density at each frequency when compared to the low power/narrowband or CW test technique, the SPTT transfer function data processing method prevents corruption of the transfer function due to noise.
As part of the SPTT transfer function data processing, a xe2x80x9ccoherence functionxe2x80x9d is computed. The coherence function is an indicator of the correlation between the two measurements (input and output). Since random noise is by definition uncorrelated, the coherence function allows measurements that are corrupted by noise to be identified and rejected, as well as being able to identifying good quality data. Therefore, excitation power levels can be significantly reduced without unknowingly compromising measurement quality.
Ambient signals, such as man-made radiators (TV and radio stations), can be used as the system excitation, which is a feature that is unique to LPWB. Transfer function measurements that can be made without active excitation signals are the best type of non-interfering measurements because no external, potentially interfering signals are produced by the instrumentation system. The coherence function makes ambient excitation feasible because it can identify and isolate the correlated ambient man-made radiators from other uncorrelated random noise sources.
The advantages of the LPWB inventive method over the impulse technique are:
a) Reduced equipment cost due to low-power excitation levels;
b) Reduced equipment cost due to non-impulse excitation;
c) Reduced health risks because low-power levels are used;
d) Reduced interference. with other systems due to the low-power of the excitation signal;
e) The maximum test frequency is not limited because of the up-conversion/down-conversion techniques;
f) Noise corrupted data is indicated by the coherence function and may be readily rejected; and,
g) Ambient excitation can be used for transfer function derivation.
The advantages of this inventive method over the CW technique are:
a) Decreased testing time due to the wideband nature of this method;
b) Reduced interference with other systems due to the low-power and noise-like nature of the excitation signal;
c) Vital transfer function information will not be missed due to the continuous frequency coverage of the wideband nature of this technique;
d) Noise corrupted data is indicated by the coherence function and may be readily rejected; and,
e) Ambient excitation can be used for transfer function derivation.
In this invention, data acquisition and processing are performed in a distributed versus centralized manner. Receivers are installed near each location in the system under test where a response measurement is desired. Because the invention uses wideband modulated signals, the signal generator used in the prior art is replaced with a waveform synthesizer. The excitation and system responses are sensed using appropriate current or voltage probes and the sensor signals are measured at the test point location rather than relayed to a central location for measurement, as is the case with the prior art. The measured signals are down-converted, digitized and relayed to a centralized test control location in digital, versus wideband analog, form. A typical transfer function test setup using the invention is shown in FIG. 3. The waveform synthesizers and receivers are centrally controlled such that all transmitted and received signals are time and frequency synchronized.
The advantages of this inventive method over the centralized data acquisition and processing apparatus are:
a) Data can be relayed from: the test point location to the central processor using low cost digital telemetry links, avoiding the dynamic range and high frequency limitation problems encountered by the analog telemetry links;
b) The dynamic range of the receiver, typically 110 dB or more, far exceeds the 40 dB dynamic range of analog telemetry links;
c) Automated Gain Control (AGC) are built-into each receiver and transparent to the telemetry link operation. The telemetry""s digital signals have a constant amplitude and thus do not require constant adjustments when measuring different signal strengths;
d) Digital relay links are more reliable and less costly than their wideband analog counter-parts;
e) Receivers can be placed at greatly disbursed locations, on the order of kilometers, which is impractical with the prior art;
f) The number of receivers that can reliably simultaneously measure frequency and phase locked responses at various test points (locations) within the system under test is virtually unlimited. Increasing the number of simultaneous data channels with the prior art is cost prohibitive; and,
g) All waveform synthesizer and receiver modules in the apparatus are time and frequency synchronized and, as a, group, effectively behave as a single instrument. Synchronization of prior art instrumentation systems are cumbersome if not impossible.
Accordingly, the main object of this invention is a method of measuring transfer functions using low-power/wideband excitation and a distributed apparatus for practicing the method. Other objects of the invention include a method of measuring transfer functions using wideband excitation; a method of measuring transfer functions with an indication of measurement quality; a method of measuring transfer functions with the capability of rejecting corrupted data; a method of measuring transfer functions under conditions where environmental (ambient) excitation can be utilized; a method of measuring transfer functions without interfering with other electronic devices; a method of measuring transfer functions without causing a health; risk; a method of measuring transfer functions over wide bandwidths without measuring one individual frequency at a time; and a method for implementing a modular distributed instrumentation architecture.
Still further objects of this invention are an unique general purpose physically distributed apparatus for using low-power, wideband radiation to excite a system and determining system""s responses to the excitation; comprised of a waveform synthesizer, for transmitting the input low-power wideband radiation to excite a system; multiple receivers, for measuring the system input and output; a digital computer, for controlling the waveform synthesizer and receiver operations and calculating the system characterizations, such as system transfer functions, from the measured data; and a digital fiber optics telemetry system to facilitate communication and synchronization between the physically distributed waveform synthesizers, receivers, and the computer.
These and other objects of the invention may be determined by reading the description of the preferred embodiments along with the drawings attached hereto. The scope of protection sought by the inventor may be gleaned from a fair reading of the claims that conclude this specification.